In typical commercial reproduction apparatus (electrographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a primary imaging member (PIM) such as a photoreceptor used in an electrophotographic printing apparatus. While the latent image can be formed on a dielectric PIM by depositing charge directly corresponding to the latent image, it is more common to first uniformly charge a photoreceptive PIM member. The latent image is then formed by area-wise exposing the PIM in a manner corresponding to the image to be printed. The latent image is rendered visible by bringing the primary imaging member into close proximity to a development station. A typical development station may include a cylindrical magnetic core and a coaxial nonmagnetic shell. In addition, a sump may be present containing developer which includes marking particles, typically including a colorant such as a pigment, a thermoplastic binder, one or more charge control agents, and flow and transfer aids such as submicrometer particles adhered to the surface of the marking particles. The submicrometer particles typically include silica, titania, various lattices, etc. The developer also typically includes magnetic carrier particles such as ferrite particles that tribocharge the marking particles and transport the marking particles into close proximity to the PIM, thereby allowing the marking particles to be attracted to the electrostatic charge pattern corresponding to the latent image on the PIM, thereby rendering the latent image into a visible image.
The shell of the development station is typically electrically conducting and can be electrically biased so as to establish a desired difference of potential between the shell and the PIM. This, together with the electrical charge on the marking particles, determines the maximum density of the developed print for a given type of marking particle.
The image developed onto the PIM member is then transferred to a suitable receiver such as paper or other substrate. This is generally accomplished by pressing the receiver into contact with the PIM member while applying a potential difference (voltage) to urge the marking particles towards the receiver. Alternatively, the image can be transferred from the primary imaging member to a transfer intermediate member (TIM) and then from the TIM to the receiver.
The image is then fixed to the receiver by fusing, typically accomplished by subjecting the image bearing receiver to a combination of heat and pressure. The PIM and TIM, if used, are cleaned and made ready for the formation of another print.
A printing engine generally is designed to generate a specific number of prints per minute. For example, a printer may be able to generate 150 single-sided pages per minute (ppm) or approximately 75 double-sided pages per minute with an appropriate duplexing technology. Small upgrades in system throughput may be achievable in robust printing systems. However, the doubling of throughput speed is mainly unachievable without a) purchasing a second reproduction apparatus with throughput identical to the first so that the two machines may be run in parallel, or without b) replacing the first reproduction apparatus with a radically redesigned print engine having double the speed. Both options are very expensive and often with regard to option (b), not possible.
Another option for increasing printing engine throughput is to utilize a second print engine in series with a first print engine. For example, U.S. Pat. No. 7,245,856 discloses a tandem print engine assembly which is configured to reduce image registration errors between a first side image formed by a first print engine, and a second side image formed by a second print engine. Each of the '856 print engines has a seamed photoreceptive belt. The seams of the photoreceptive belt in each print engine are synchronized by tracking a phase difference between seam signals from both belts. Synchronization of a slave print engine to a main print engine occurs once per revolution of the belts, as triggered by a belt seam signal, and the speed of the slave photoreceptor and the speed of an imager motor and polygon assembly are updated to match the speed of the master photoreceptor. Unfortunately, such a system tends to be susceptible to increasing registration errors during each successive image frame during the photoreceptor revolution. Furthermore, given the large inertia of the high-speed rotating polygon assembly, it is difficult to make significant adjustments to the speed of the polygon assembly in the relatively short time frame of a single photoreceptor revolution. This can limit the response of the '856 system on a per revolution basis, and make it even more difficult, if not impossible, to adjust on a more frequent basis.
In general, the timing offset of the first and second engines are determined by paper transport time from image transfer in the first engine to the image transfer in the second engine. If the sheet is inverted between the engines, the transport time can be a function of the receiver length. In order to obtain sufficient timing latitude to compensate for varying receiver sheet sizes, one could run the inverter assembly at a very high rate of speed to minimize the effects of receiver size. Alternatively, one can use the maximum size image frame for all receiver sizes. However, this would significantly reduce productivity.
Color images are made by printing separate images corresponding to an image of a specific color. The separate images are then transferred, in register, to the receiver. Alternatively, they can be transferred in register to a TIM and from the TIM to the receiver or they may be transferred separately to a TIM and then transferred and registered on the receiver. For example, a printing engine assembly capable of producing full color images may include at least four separate print engines or modules where each module or engine prints one color corresponding to the subtractive primary color cyan, magenta, yellow, and black. Additional development modules may include marking particles of additional colorants to expand the obtainable color gamut, clear toner, etc., as are known in the art. The quality of images produced on different print engines can be found to be objectionable if produced on different print engines even if the print engines are nominally the same, e.g. the same model produced by the same manufacturer. For example, the images can have slightly different sizes, densities or contrasts. These variations, even if small, can be quite noticeable if the images are compared closely.
Problems that appear to be due to the print engines being out of synchronization but arising from transient occurrences in a digital print engine comprising coupled digital print engines can occur if the speed of any of the several modules varies, as can occur due to torque disturbances, power line voltage or frequency variations, etc. This can be especially problematic if there is a signal to the logic and control unit (LCU) indicating that a speed variation is sufficiently large or if a signal is reported that a module, operating within its speed specifications but near the end the limit of that specification, had been subjected to a small incremental change in module speed resulting in that module operating outside its specifications. Either of these cases can result in unnecessary changes in the time delay needed to allow the print engines to be synchronized and may actually result in the entire print engine being shut down. These error signals may either indicate a problem that needs correction or simply be a transient or simply data spikes that are not indicative of a problem needing rectification. Moreover, by correcting for the perceived mismatch in timing, the control unit can actually drive the print engines out of synchronization. It is therefore important to verify that a timing error is indeed due to the print engines being out of synchronization rather than due to a fluctuation in the synchronization.
It is clearly important that certain image quality attributes, including size, print density, and contrast, match for prints made on separate print engines if those prints are subject to close scrutiny, as would be the case when a print made on a receiver sheet is produced on separate print engines. Specifically, the reflection density and the contrast of the prints need to closely match or the prints will be found to be objectionable to a customer. Even prints produced on two nominally identical digital printing presses such as electrophotographic printing presses described herein can vary in density and contrast due to variations in the photo-response of the PIM, variations in the charge or size of the marking particles, colorant dispersion variations within the batches of marking particles used in the separate engines, etc. It is clear that a method is needed to allow comparable prints to be produced on a plurality of engines.